We studied nineteen total knee replacements and thirty-three total hip replacements with simultaneous biplane radiography to measure in vivo micromotion at the cement-bone interface. Spherical cobalt-chromium markers were embedded in the cement and cortical bone. Relative motion of the cement-markers was measured with respect to the markers in the cortical bone. Reversible displacement (relative motion during a change from non-weight-bearing to weight-bearing) and migration (relative motion over time from one non-weight-bearing study to another) were calculated. The resolution of the measuring system was 0.2 millimeter. The range for symptomatic reversible displacement was 0.4 to 4.5 millimeters, while that for asymptomatic reversible displacement was 0.3 to 1.9 millimeters. All reversible displacement of less than 0.4 millimeter was asymptomatic. Migration of as much as 2.1 millimeters occurred without concomitant reversible displacement. All radiolucent lines correlated with measured reversible displacement. Half of the patients who were evaluated two weeks postoperatively had measurable reversible displacement. CLINICAL RELEVANCE: Our findings were based on a small series from a selected population. The incidence of measured reversible displacement (75 per cent) was based on in vivo techniques and included symptomatic and asymptomatic reversible displacement; therefore, it was higher than the incidence of clinical loosening (9.6 per cent in our series). Simultaneous biplane radiography may become an important adjunct in the postoperative management of patients with a total joint replacement.